setting ctx to null after close

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/api/java/Context.java

@@ -4133,5 +4133,6 @@ public void close()
         synchronized (creation_lock) {
             Native.delContext(m_ctx);
         }
+        m_ctx = null;
     }
 }